1. Field of the Invention
The present invention relates to a pump-off controller and a position synthesizer of a well pumping unit of the type having a walking beam which is cyclically operated by a motor for raising and lowering a polish rod of a borehole pump.
2. Description of the Prior Art
Pump-off controllers are employed for shutting down the pump of an oil well if a pumped off condition has occurred and to turn the pump back on after a reasonable waiting time. By pumped off is meant that the fluid level in the well drops such that the pump is not lifting as much fluid and hence, the amount of work done by the pump decreases. Continued pumping under pumped off conditions may result in damage to the equipment. Thus, the pump is shut down for a period sufficient to allow the fluid level to raise to a desired pumping level. The pump-off controllers also shut down the well in the event of a malfunction of the pump such as a parted rod.
In measuring the characteristics of a pumping oil well and for use by the controller it is necessary to determine the position vs. time of the polished rod and to transform this motion to an electric analog signal. For some years the only method used to generate an electric analog of position has been that of mechanically coupling the rod or beam to a potentiometer. This method is subject to frequent failure of the mechanical coupling and wear of the potentiometer.
As disclosed in the October 1979, issue of the Journal of Petroleum Technology, load vs. displacement measurements of the polished rod have been obtained and the area within the resulting curve used to control pump-off. This method of controlling pump-off has resulted in problems since it is sensitive to "high fluid level" distortion of the load position curve and to "bellying," a distortion sometimes associated with pump-off.